This invention relates to a print hammer having wear-resistive contactor in a dot printer.
A dot printer operates to print characters on a recording sheet with small points, namely, dots which are formed on the recording sheet by several dotting members arranged adjacent to one another. The dotting members are actuated to hammer the recording sheet through a carbon ribbon with suitable timing to form dots on the recording sheet. If the dot printer is employed, it is unnecessary to provide a number of pieces of type in advance, and it is possible to print characters using only a few dotting members. Thus, a dot printer has been extensively employed in a variety of printing systems.
In one prior art dot printing system, print hammers adjacent to one another in a line are arranged in parallel either with the dotting surface of a recording sheet moving vertically or a copying material moving horizontally. Among these print hammers, only the print hammers necessary for printing a contemplated character are operated to print dots thereby forming the character. This operation is repeatedly carried out to print a line of characters.
The selection of print hammers and the timing of operating the print hammers for printing a particular character are stored in a separate memory device, so that dots can be printed according to instructions in a very short time. Thus, printing can be achieved at a high rate of about 200 dots/second.
In general, the above-described print hammer has a dotting contactor which is secured to a leaf spring. The leaf spring serves as the armature of an electromagnet provided behind it. Since the contactor should satisfactorily strike an ink ribbon or a recording sheet to print a dot, the contactor has a small contact end (for instance, 0.2 to 0.8 mm) in diameter and its height is of the order of 2 to 5 mm. The electromagnet pulls the leaf spring to a position where the leaf spring is relatively bent from its rest position, with its support a fulcrum. The electromagnet releases the leaf spring in response to an instruction signal from the above-described memory device to thereby print a dot by the utilization of the elastic force of the leaf spring.
Such a conventional protruding contactor is extremely disadvantageous in that, since it is made of steel, it is significantly worn, i.e., the length of the contactor is reduced. As a result the hammer swinging distance is increased, or the contact surface of the contactor is rounded, i.e., the edge of the contact surface is removed. As a result the printed dot is not sharp and the character formed is not clear.
In order to overcome the difficulty where the contactor is easily worn out, the contactor has been made of sintered hard alloys such as tungsten carbide-cobalt alloys. However, a contactor made using these materials is poor in rigidity; that is, it is liable to be cracked or broken. Furthermore, the contactor is low in corrosion resistance when encountering ink or the like. Accordingly, steel that is unsatisfactory in wear-resistance and corrosion resistance is still used to manufacture the contactor, although the problems remain largely unsolved.
The inventor has conducted intensive research to overcome the above-described difficulties. As a result of the research, the inventor has found that if the contactor is made of a particular alloy, then, the wear-resistance is improved, the contactor will not be broken, the service life is increased, troublesome print hammer replacement can be eliminated, and the reliability of the dot printer can be improved, and accomplished.